During the Cretaceous, 139-65 million years ago, shallow seas covered
much of the southern United States. These tropical waters were
productive–giving rise to tiny marine plankton with carbonate skeletons
which overtime accumulated into massive chalk formations. The chalk,
both alkaline and porous, lead to fertile and well-drained soils in a
band, mirroring that ancient coastline and stretching across the now
much drier South....
Over time this rich soil produced an amazingly productive agricultural
region, especially for cotton. In 1859 alone a harvest of over 4,000
cotton bales was not uncommon within the belt. And yet, just tens of
miles north or south this harvest was rare. Of course this level of
cotton production required extensive labor...
As Washington notes further in his autobiography, “The part of the
country possessing this thick, dark, and naturally rich soil was, of
course, the part of the South where the slaves were most profitable, and
consequently they were taken there in the largest numbers."